Molten hydroxide membrane for separation of acid gases from emissions

ABSTRACT

In one embodiment, a method for separating acidic gases from a gas mixture includes exposing the gas mixture to a separation membrane at an elevated temperature, where the separation membrane includes a porous support and at least one molten alkali metal hydroxide disposed within pores of the porous support.

RELATED APPLICATIONS

The present application is a division of, and claims priority to, U.S.patent application Ser. No. 15/159,681, filed May 19, 2016 (published asU.S. Patent Gazette Publication No. US 2017/0333834 A1 on Nov. 23,2017).

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The presently disclosed inventive concepts relate to carbon capture andseparation (CCS), and more particularly to dual-phase separationmembranes for post-combustion carbon capture and separation.

BACKGROUND

Greenhouse gas emissions, and particularly carbon dioxide emissions, arean increasing concern in the context of environmental health and as acontributor to global climate change. As such, much effort is currentlyspent developing ways to reduce or eliminate carbon dioxide emissionsfrom important modern processes, particularly industrial processes.

A well-known and predominant source of carbon dioxide emissions in themodern industrial economy arises from power production. Accordingly,this sector has been the focus of efforts to reduce or eliminate carbondioxide emissions. One typical approach to accomplish this objective isthe use of a separation membrane or amine-based separation solution tocapture carbon dioxide after combustion but prior to releasing exhaustinto the atmosphere. Some solutions instead attempt to capture carbondioxide pre-combustion, but have met with limited success, and areassociated with prohibitively high costs due to the need to integratelegacy systems with complex fuel-conversion processes.

Focusing now on post-combustion capture techniques, many conventionalseparation systems employ a solid phase membrane, such as a polymer orceramic; a solution of organic amines (e.g. monoethanolamine, or MEA)solvated in water; or a dual phase molten carbonate/ceramic membrane toaccomplish separation of carbon dioxide.

Typical polymer-based membrane separation systems generally separategases through size effects or chemical effects. Size exclusion membraneseffectively separate carbon dioxide from hydrocarbons but haveprohibitively low selectivity for separating carbon dioxide from othergases, such as molecular nitrogen, as needed for the remediation of fluegases common in the targeted application to industrial sources ofgreenhouse gas emissions. Chemical exclusion membranes separate carbondioxide from molecular nitrogen with high selectivity, but atprohibitively slow rates. Polymer membranes are also troubled by limitedtemperature ranges (i.e. <250 C) and are prone to fouling by gases (e.g.sulfur and nitrogen oxides) and/or particulate matter in the flue gas.

Conventional oxygen separation membranes employ solid-phase ceramicseparation systems which separate molecular oxygen from molecularnitrogen before combustion, instead of separating carbon dioxide frommolecular nitrogen after combustion. These membranes require operationat temperatures of 800-1000 C, which is much higher than the 300-700 Ctemperature range at which targeted sources of greenhouse gas emissionsoperate, the temperature at which flue gas exits the combustion chamber(prior to the heat exchanger). Preheating air to this high temperaturerange for removing the 20% fraction of molecular oxygen is energeticallyexpensive, and reduces the efficiency of the power production capabilityof the emission source. This system is also reactively slow (1×10⁻¹¹ to1×10⁻⁸ mol s⁻¹ cm⁻² in the 800-1000 C range) because the oxygen musttransition through the solid phase as oxide (O²⁻) ions.

Traditional liquid phase treatment, such as amine gas treatment involvesusing an organic amine in a water solvent to capture cold (<80 C) carbondioxide in an absorber from the flue gas of the emission source, e.g. afossil fuel power plant. The carbon dioxide is later released in aregenerator with the application of heat energy. This system consumes25-40% of the power plant's energy through exchanged and consumed heatenergy. The infrastructure costs for an absorber, regenerator, etc., arealso substantial. Together, the energy and infrastructure costs for thissystem are expected to double the cost of electricity produced by theemission source. Unfortunately, the use of other materials for carbondioxide adsorption does not significantly change the energy orinfrastructure costs.

Existing dual phase separation membranes, such as carbonate-ceramicmembranes, separate carbon dioxide from molecular nitrogen using both amolten (liquid) carbonate phase and a solid ceramic phase. The solidceramic phase serves both as a porous, solid structural support for themolten carbonate phase as well as an oxide ion conductor via the sameconduction process involved with oxygen separation membranes. The moltencarbonate phase transports carbon dioxide across the membrane ascarbonate ions (CO₃ ²⁻), while oxide conduction in the oppositedirection to carbonate conduction is required to maintain oxygen andcharge balance. The conduction of O²⁻through the solid phase is muchslower than the relatively fast conduction of carbonate through theliquid phase and so the whole system is rate-limited by the slow rate ofoxide conduction.

While mixtures or eutectics of lithium, sodium, and potassium carbonatecan reach melting temperatures as low as 400-500 C and can conductcarbonate ions in this temperature range, this system nonethelessrequires much higher operational temperatures because of the additionalrequirement for oxide conduction. Similar to oxygen separationmembranes, the application of the carbonate-ceramic dual phaseseparation membrane is limited to very high temperatures (>700 C) due tothe substantial thermal energy required to move oxide ions through thesolid phase. The concurrent problems with a dependence upon oxideconduction through a solid phase render the separation processprohibitively slow and requires operation temperatures which are toohigh for effective separation of carbon dioxide from hot flue gas.

Accordingly, it would be beneficial to provide systems and techniquesfor separating carbon dioxide gas from emissions sources such as fluegas of fossil fuel power plants that are capable of operating at theambient temperature of the flue gas as emitted from the source with asufficient reaction rate to effectively separate the carbon dioxidewithout consuming substantial power from the source and withoutassociated infrastructure costs incurred by the existing techniques andsystems described above.

SUMMARY

In one embodiment, a method for separating acidic gases from a gasmixture includes exposing the gas mixture to a separation membrane at anelevated temperature, where the separation membrane includes a poroussupport and at least one molten alkali metal hydroxide disposed withinpores of the porous support.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription read in conjunction with the accompanying drawings.

FIG. 1A depicts a separation mechanism based for molten carbonateseparation membranes, according to conventional approaches.

FIG. 1B depicts a separation mechanism based for molten hydroxideseparation membranes, according to one embodiment of the presentlydisclosed inventive concepts.

FIG. 2A is a simplified schematic of a separation membrane, according toone embodiment.

FIG. 2B depicts a simplified schematic of a separation membrane havingformed thereon conductive electrodes, according to another embodiment.

FIG. 3 is an image of a porous magnesium oxide support structure,according to one embodiment.

FIG. 4 is a flowchart of a method for separating acidic gases fromemissions, according to various embodiments.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

As also used herein, the term “about” when combined with a value refersto plus and minus 10% of the reference value. For example, a length ofabout 1 μm refers to a length of 1 μm±0.1 μm.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofthree dimensional porous separation membranes and/or related systems andmethods of making the same.

In one general embodiment, a separation membrane includes: a poroussupport structure; and at least one alkali metal hydroxide disposedwithin pores of the porous support structure.

In another general embodiment, a method for separating acidic gases froma gas mixture includes exposing the gas mixture to a separation membraneat an elevated temperature, where the separation membrane includes aporous support and at least one molten alkali metal hydroxide disposedwithin pores of the porous support.

In a conventional power plant (and various other types of internalcombustion engines), exhaust from combustion of fuels typically includesgreenhouse gases, such as CO_(x), and offensive acid gases such asNO_(x), and SO_(x). The exhaust gases are exist at a high temperature,e.g. in a range from 300-700 C, and may be utilized to perform thermalwork, e.g. by converting water to steam for harvesting energy from thecombustion reaction.

There is a need for more efficient and less expensive carbon dioxideseparation technology for carbon capture and sequestration (CCS). Thepresently disclosed inventive concepts provide a carbon dioxideseparation membrane which is so energy and cost efficient that itoutcompetes other technologies and is economically implementable. Theinventive systems and techniques leverage reversible carbon dioxidesolubility in molten hydroxide, e.g. potassium, sodium, and/or lithiumhydroxide (KOH, NaOH, LiOH) electrolytes.

Interestingly, the proposed mechanism counters conventional expectationsthat molten hydroxide electrolytes will completely and irreversiblyconvert to the solid phase carbonate according to the reaction shown inequation (1), below. Instead, at temperatures above 250 C, the reactionis actually reversible as shown in equation (2).2KOH+CO₂→K₂CO₃+H₂O  (Equation 1)2KOH+CO₂↔K₂CO₃+H₂O  (Equation 2)

Reaction (1) becomes reversible in the molten system (e.g. >30 Mpotassium hydroxide) at elevated temperatures (e.g. >250° C.) as long assufficient water is present in the system. The presence of water shiftsthe equilibrium of reaction (1) to the left. Molten potassium hydroxideretains molar concentrations of water up to 400-600° C., depending onsteam pressure. Advantageously, water is a combustion byproduct whichoccurs in concentrations similar to carbon dioxide in flue gases atsufficient amounts to maintain the reversibility of the reaction asshown in equation (2).

Acid/base chemistry separates acid gases from non-acid gases. The moltenhydroxide system absorbs carbon dioxide, nitrogen dioxide, and sulfurdioxide acid gases by forming CO₃ ²⁻, NO₃ ⁻, and SO₄ ²⁻, respectively,with molar solubilities. The solubility of the acid gases and therelative insolubility of molecular nitrogen, oxygen and hydrogen gasesmeans the membrane can selectively remove the greenhouse gases from theflue gas.

A pressure gradient may be utilized for pulling solvated gases through amembrane. The removal of the dissolved gases from one side of themembrane causes a concentration gradient which moves the gases from highpressure (the flue gas side) to low pressure (acid gas removal side)through diffusion. The rate of acid gas transport across the membrane isa function of solubility, diffusion coefficients, membrane thickness,and pressure gradient.

As noted above, existing techniques for carbon capture function by usingeither low-efficiency separation membranes (e.g. molten carbonate) or anamine-based solution to separate carbon dioxide from exhaust. Thesetechniques are limited by the rate of oxide transport through solidphase (in the case of molten carbonate membranes) and/or consumesignificant energy to accomplish separation. The presently disclosedinventive concepts overcome these limitations, and offer a broader rangeof applicability, through the use of molten hydroxide to facilitatecapture and separation of acidic gases.

Mechanistically, the differences between previous separation membranefunction and the presently disclosed hydroxide-based separation areshown in FIGS. 1A-1B, respectively.

The known mechanism shown in FIG. 1A involves selective capture ofcarbon dioxide at an inlet of the separation membrane (left side of FIG.1A) and subsequent conversion to a carbonate ion for transport throughthe molten carbonate phase of the membrane and ultimate release from anoutlet of the membrane (right side of FIG. 1A). To balance theequilibrium of this exchange, oxide ions are transported in the oppositedirection. As noted above, the oxide ion travels through the solid phaseof the membrane, and is thus a rate-limiting factor in the separationprocess. In addition, this solid phase oxide transport requires veryhigh operating temperatures, e.g. above 700 C, which undesirably limitsthe applicability and efficiency of the separation process.

Accordingly, a new mechanism is proposed in FIG. 1B, based on discoverythat carbon dioxide and other similar acidic gases are reversiblysoluble in a molten hydroxide phase. In operation, the carbon dioxide(or other acidic gas as described herein) is introduced to theseparation membrane at the inlet (left side of FIG. 1B) and converted toa carbonate for transport through the molten hydroxide phase of themembrane and ultimately released from the outlet of the membrane (rightside of FIG. 1B). In contrast to the known mechanism of FIG. 1A, thistransport is balanced via conduction of hydroxide and/or oxide ions inthe opposite direction, which advantageously occurs within the moltenhydroxide phase as opposed to the solid phase.

Accordingly, the presently disclosed inventive concepts avoid therate-limiting and high temperature-dependent transport of oxide ionsthrough a solid phase of the membrane, conferring significant advantagesto separation of acidic gases such as carbon dioxide. The particularadvantages and mechanisms will be described in further detail below,according to various exemplary embodiments.

In one approach, a molten hydroxide carbon dioxide separation membraneseparates carbon dioxide from molecular nitrogen with a molten hydroxideelectrolyte, such as molten mixtures of alkali metal hydroxides likepotassium, sodium, cesium, rubidium, and/or lithium. The moltenhydroxide system serves to conduct both carbonate and oxide in theliquid phase. The molten hydroxide advantageously absorbs and solvateslarge concentrations (e.g. up to 50 wt %) of carbon dioxide ascarbonate. The molten hydroxide can also conduct oxide ions quicklythrough the liquid phase as hydroxide, hydronium, and/or protons (H+).

The conduction of hydroxide and protons may occur, and preferably occursmuch more rapidly than conduction of carbonate across the membrane. As aresult the separation process is rate limited by carbonate masstransport rather than the transport of oxide ions through a solid phase.Accordingly, the molten hydroxide membranes disclosed herein separateacidic gases approximately 100-10,000 times faster at a giventemperature than the molten carbonate membranes because the separationrate is limited by the transport of carbonate in the liquid phase ratherthan by the rate of oxide transport through the solid phase.

For instance, as discussed further herein, in a range from approximately350 to about 700 C, in various approaches, the 10²-10⁴ fold separationrate improvement may be accomplished. As will be appreciated by personshaving ordinary skill in the art upon reading the present disclosure, athigher temperature ranges even higher separation rates may be achieved.

A difference in partial pressure of CO2 between the feed and permeatesides of the membrane is advantageous to transport dissolved CO2 acrossthe membrane. A pressure gradient between the feed and permeate side,and/or utilizing a sweep gas on the permeate side to lower CO2 partialpressure may be utilized to facilitate transport of gases through themembrane in the context of the present disclosures.

The hydroxide melts are relatively viscous, yet exhibit exceptionallyhigh ionic conductivities for ionic species such as carbonate, nitrateand sulfate, which can be transported quickly through the melt. Thepartial pressure across the membrane is a driving force for net movementof carbon dioxide and other acid gases through the membrane. The rate atwhich the gases move across the membrane are a function of solubility,diffusion coefficients, membrane thickness, concentration of watervapor, and partial pressure gradient. The solubility and diffusioncoefficients are predominantly controlled by gas temperature andcomposition of the molten electrolyte. The membrane thickness ispreferably minimized to facilitate rapid mass transport withoutsacrificing structural integrity required to support the membrane underthe pressure gradient.

In various embodiments, and consistent with operational conditions ofenvironments such as flues of combustion power plants, the pressuregradient to which the membrane may be exposed, and thus must withstand,is in a range from approximately 0-20 atmospheres for transporting thegases across the membrane.

As will be understood by persons having ordinary skill in the art uponreading the present disclosures, a high pressure gradient coulddisadvantageously pull the hydroxide melt through and out of the supportmatrix. This is one reason why using a sweep gas such as steam isadvantageous, in various embodiments and as described in further detailbelow. In particular, using a sweep gas may advantageously push theequilibrium shown in equation 2 to the left and lower the partialpressure contribution of species to be removed from the gas mixture (onthe permeate side).

Accordingly, the presently disclosed inventive separation membranes aredesigned to retain the molten hydroxide phase within a solid supportbased on capillary action. As such, the support structure is preferablya porous matrix such as an aerogel, and even more preferably ischaracterized by pores having a diameter in a range from approximately75 nm to about several (e.g. 10) microns. As will be understood bypersons having ordinary skill in the art upon reading the presentdescriptions, larger pore size is desirable to facilitate rapid masstransport through the membrane, but pores must be sufficiently small toretain the molten hydroxide phase within the matrix when subjected tothe pressure gradient generated within the separation environment.

In some embodiments, it is advantageous to include use a membrane withvarying pore size throughout the cross-section, e.g. a gradient in poresize. For example large pores may be included in a region proximate tothe inlet of the membrane, and may gradually become smaller toward anoutlet of the separation membrane. Such embodiments allow thickmembranes that have the required mechanical strength without sacrificingtransport or liquid retention. In alternative approaches, the gradientmay be configured such that pores having a smallest average diameter arelocated in a central region of the separation membrane (measuredaccording to a direction along which ions migrate through the membrane)with larger pores located in proximity to each outer face of themembrane.

In several experiments, it has been determined that a pressure on theorder of 20 atmospheres or more is required to overcome capillary actionto remove water from a 200 nm SiO₂ pore. Accordingly, the presentlydisclosed inventive separation membranes are preferably characterized bypores with an average diameter of approximately 200 nm, in one approach.Since hydroxide exhibits strong electrostatic attraction, it is expectedthat larger average pore sizes may be feasible, advantageouslyfacilitating increased mass transport across the separation membrane.For instance, in one embodiment pores with an average diameter ofapproximately 500 nm may be utilized without sacrificing structuralintegrity of the membrane under a pressure gradient of 5-20 atmospheres.Indeed, in some approaches, it is possible to separate CO2 with nopressure gradient at all, by driving the separation using aconcentration gradient with respect to CO₃ ²⁻ions across the membrane.Alternatively, the reaction may be driven by use of a sweep gas to applya pressure gradient of 0 atmospheres across the gradient, by effectivelybalancing pressure across the membrane.

Pores may have an average diameter in a range from about 10 nm to about1 mm, from about 100 nm to about 100 um, from about 100 nm to about 10um, from about 200 nm to about 5 um, or in a range from about 200 nm toabout 1 um, in various embodiments. As will be appreciated by skilledartisans upon reading the present disclosures, the pore size is criticalto retention of hydroxide in pores of the porous support structure undera given pressure gradient.

Accordingly, pore size should be controlled according to the stress towhich the separation membrane will be subjected in operation for theparticular application in which the membrane will be employed. As willbe further appreciated by skilled artisans upon reading thesedisclosures, therefore the pore size may be determined based onoperating conditions (particularly temperature and pressure) under whichthe membrane will be used and mechanical and/or electrostatic propertiesof the materials from which the membrane is formed.

For example, in one embodiment porous materials based on ZrO₂ have beenexperimentally determined to retain 62 wt % KOH loading at 400 C over aduration of >48 hours. The heating, in one approach, includes incubationover a period of 192 hours, with four cycles of heating to 400 C andcooling to room temperature (e.g. 23-27 C). The pore size for thisembodiment was determined to be in a range of approximately 0.1-0.5 um,and retention was accomplished under conditions that did not includeapplication of a pressure gradient.

Without wishing to be bound to any particular theory, the inventorspostulate this unexpected and superior retention is attributable tobetter-than-expected wettability of hydroxide in ceramic materials suchas cerium oxide, zirconium oxide, silicon carbide, and equivalentsthereof that would be appreciated by a person having ordinary skill inthe art upon reading the present disclosures.

In more approaches, and preferable in some applications to reduce energyconsumption associated with separation of offensive and/or acidic gasesfrom gas mixtures such as flue gas, the presently disclosed inventiveseparation membranes and processes may accomplish separation withoutapplying a pressure gradient via use of a sweep gas. For instance, inone embodiment water vapor may be passed across the permeate side of themembrane (opposite the side where offensive and/or acidic gases arepresent in high concentration, e.g. in the flue). Optionally, butadvantageously, the sweep gas may apply pressure to the permeate side ofthe membrane.

Using a sweep gas is an attractive embodiment for several reasons.First, the water pushes the equation (2) equilibrium over to favor CO₂release on the permeate side of the membrane. Second, CO₂ separates at ahigher rate than without sweep gas utilization, because the sweepingaction maintains a larger CO₂ gradient across the membrane than avacuum, with less energy. Third, if the steam pressure applied to thepermeate side of the membrane is approximately equal to the pressureapplied to the membrane by the gas mixture (e.g. flue side), there is nodifference in total pressure across the membrane, which significantlyrelaxes the materials challenges for capillary action (pore size) andmechanical strength. Fourth, steam is already present and available invarious suitable applications, such as power plants (where steam is usedin the heat exchanger), obviating the need to provide external sourcesof steam and further improving energy efficiency of separation. Fifth,water vapor can be easily separated from the CO₂ after gas separation(e.g. via condensation), which further allows advantageous harvesting ofheat from the water vapor.

In one embodiment, an exemplary separation membrane 200 is shownaccording to a simplified schematic as represented in FIG. 2A. Theseparation membrane 200 includes a porous support structure withhydroxide disposed in the continuous pore system defined by the voids ofthe porous support. The separation membrane includes an inlet 202 and anoutlet 204 through which acidic gases are transported via the hydroxideunder operating conditions including an elevated temperature in a rangefrom approximately 300-700 C and a pressure gradient across the membranein a range from approximately 5-20 atmospheres. In other approaches,separation of oxide gases such as CO₂, SO₄, and/or NO₃ may beaccomplished without the use of a pressure gradient, relying insteadsolely on a concentration gradient across the membrane to facilitatemass transport thereacross.

In preferred approaches, the porous support structure comprises a systemof continuous pores connecting the inlet 202 of the separation membraneto the outlet 204 of the separation membrane, thus enabling efficientmass transport thereacross. The separation membrane 200 may be formed inany suitable configuration, and preferably is characterized bydimensions and a shape suitable to fit within an existing structure ofthe emissions source (e.g. within a heat exchanger of a power plant,cement factory, etc.) and separate a high-pressure, acidic gas-richenvironment from a low-pressure, acidic gas-poor environment (e.g. acapture facility, tank, line, etc.). While the configuration shown inFIGS. 2A-2B is characterized by a circular configuration, other shapesmay be implemented without departing from the scope of the presentdisclosures. In various experimental embodiments, the support structuremay be formed and have a cross sectional area on the order of 100 cm²,e.g. a 10 cm×10 cm square.

Preferably, the support structure is characterized by a surface area ina range from approximately 50 m² to approximately 1000 m² in order tomaximize mass transport through the continuous pore system thereof. Asdescribed in further detail below, surface area may be reduced whileachieving equivalent separation via application of electric potential,e.g. an alternating current, across the separation membrane 200. Intheory, complete carbon dioxide removal for a 400 megawatt power plantmay be achieved using a surface area in a range from approximately 50 m²to approximately 1000 m² even without the application of electriccurrent, and may be accomplished with even lower surface area structuresby investing electrical energy on the order of 1-2% of the power plant'stotal production to facilitate mass transport.

In addition, the separation membrane 200 is characterized by a thicknesssufficient to withstand the pressure gradient, but minimally thick so asto facilitate rapid mass transport through the membrane. In oneapproach, a thickness of approximately 0.5 cm has been determinedsufficient to accomplish rapid transport while withstanding 5-20atmosphere pressure gradients.

In one embodiment, enhancing mass transport of the dissolved acid gasspecies across the membrane may be accomplished by using an alternatingcurrent (AC) wave. Accordingly, and as shown in FIG. 2B, severalembodiments of the presently disclosed inventive separation membrane 200may have a porous, conductive membrane 206 coupled to the inlet and/oroutlet of the separation membrane 200. The conductive membrane mayinclude a material such as graphene, activated carbon, or otherconductive material as disclosed herein and would be understood assuitable by a person having ordinary skill in the art upon reading thepresent disclosures for the purpose of serving as an electrode.

Mass transport of the dissolved acid gases across the membrane can beenhanced by causing migration in addition to diffusion because dissolvedspecies such as carbonate, nitrate, and sulfate (CO₃ ²⁻, NO³⁻, SO₄ ²⁻)are charged. Ions migrate in response to applied current/voltage. Sincepotassium hydroxide/sodium hydroxide melts have exceptionally high ionicconductivities (e.g. 1 S/cm at 300 C), an alternating current (AC) canquickly and efficiently migrate the dissolved acid gas species from highto low concentration. The application of an AC signal increases both therate and selectivity of acid gas separation from non-acid gases becausethe latter will not ionize in the molten electrolyte. Accordingly,conductive coatings may serve as electrodes for application of the ACsignal, and may be formed by coating each side of the membrane withactivated carbon (or other graphitic material) which is preferablystable, conductive, and permeable to gases.

In various embodiments, the support structure may comprise an aerogelformed of materials selected from inconel 600, grade 316 stainlesssteel, an alkaline earth oxide, yttrium doped zirconium oxide, ceriumoxide, magnesium oxide, aluminum oxide, calcium carbonate and siliconcarbide. Any other equivalent material that would be appreciated by askilled artisan upon reading the present disclosures may also beutilized without departing from the scope of the invention describedherein. The support material should be capable of withstanding high pHenvironments characteristic to molten hydroxides, high operatingtemperatures of a flue or other similar environment (e.g. 300-700 C),withstand oxidation by the residual O₂ in the flue gas, and be robust towater vapor and at such temperatures. In addition, the support materialis ideally characterized by being highly wettable by the moltenhydroxide phase.

FIG. 3 depicts an exemplary embodiment of such a support structurecomprising a porous magnesium oxide aerogel. The support structure maybe formed using any suitable technique known in the art, in variousapproaches. For instance, in one approach fabrication of a porousmagnesium oxide aerogel may be accomplished substantially as disclosedby Li, et al. in a publication entitled “Hard-templating pathway tocreate porous magnesium oxide,” Chemistry of Materials 16:5676-81(2004).

In more embodiments, the support structure may comprise other materialsas disclosed herein, and may be formulated using a modified version ofthe procedures disclosed by Li, et al., and/or may involve the use ofadditive manufacturing and/or three-dimensional (3D) printingtechniques. For instance, several embodiments may form the presentlydisclosed inventive structures via the use of a templating process incombination with known 3D printing techniques, as would be understood bya person having ordinary skill in the art upon reading the instantdescriptions.

Faster movement of acid gases across the membrane desirably minimizesinfrastructure quantity and cost of the overall separationprocess/solution. In preferred approaches, a mixture of molten hydroxide(e.g. approximately equimolar amounts of NaOH/KOH/LiOH, or amounts in arange as described by the ratios provided below, in various embodiments)provides an exceptionally ionic conductivity of 1 S/cm at 300 C eventhough the viscosity thereof is in a range of approximately 2-3centipoise. In another embodiment, the eutectic mixture may includealkali metal hydroxides according to a ratio of 4KOH:4NaOH:1LiOH. Inanother embodiment, the mixture may include alkali metal hydroxidesaccording to a ratio of 1KOH:1NaOH:4LiOH.

Applying an AC wave advantageously enhances mass transport withmigration because the acid gases are ionic (CO₃ ²⁻, NO³⁻, SO₄ ²⁻) whendissolved in the hydroxide melt. The melt's exceptionally high ionicconductivity means an AC wave can quickly and efficiently migrate thedissolved gases from high to low concentration sides of the membrane,thus improving the efficiency of the separation process.

Accordingly, various embodiments of the presently disclosed inventiveconcepts will include mixtures of hydroxides as disclosed herein havinga ratio in a range of 4:1:1 to 1:1:4, where each component is selectedfrom the exemplary hydroxides disclosed herein. Preferred embodimentsinclude ternary mixtures of KOH, NaOH, and LiOH, with mixtures includinghigher lithium concentrations being particularly preferred. In severalembodiments, the mixture may be characterized as eutectic, i.e.comprising a homogenous solid mixture of atomic and/or chemical speciesforming a joint super-lattice such that each pure component has adistinct bulk lattice arrangement. The eutectic mixture, as describedherein, melts as a whole (as opposed to individual components meltingindividually under different conditions such as temperature andpressure) at the lowest possible melting temperature over all possiblemixing ratios for the involved atomic/chemical species.

Molten hydroxide eutectics can melt as low as 150-170 C and so themembrane is operational at even lower temperatures than 300-700 C andpressures of 5-20 ATM in embodiments featuring the eutectic mixture orsimilar composition. In such embodiments, the molten hydroxide willadsorb CO₂ upon exposure to the gas mixture (e.g. flue gas) andeventually reach a steady state carbonate concentration during theseparation process. The molten separation phase can then be described ashaving an average concentration of KOH, NaOH, LiOH, K₂CO₃, Na₂CO₃,and/or Li₂CO₃, depending on the particular hydroxide species included inthe mixture. The molten separation phase will not completely convertfrom hydroxide species to carbonate species because the separationprocess will cease in the absence oxide and hydroxide conduction in theliquid phase. The molten electrolyte therefore preferably maintains asignificant average concentration of >approximately 5 mol % hydroxideduring continuous operation.

Of course, the eutectic mixtures are also operational at temperaturesand pressures existing in the flue gas at a region prior to the powerplant's heat exchanger. Advantageously, at this point the carbon dioxideis sufficiently energetic to drive the separation process withoutaddition of energy from outside sources, eliminating the need tocannibalize energy from the power plant or other emission source.

The molten hydroxide separation phase is highly selective for carbondioxide and other acid gases such as sulfates and oxides of nitrogen(SO₂ and NO_(x)), which also would be separated in ideal circumstances,but which undesirably usually foul other membrane systems. Since carbondecomposes in contact with molten hydroxide in the envisionedtemperature range of 300-700 C and the 2-5% residual O₂ found in fluegas, the presently disclosed systems can advantageously self-clean, thusavoiding the fouling experienced when using solid-phase polymer-basedseparation membranes.

The molten hydroxide system disclosed herein is the fastest ionconductor developed to-date, and is characterized by a separation ratein a range from approximately 1×10⁻⁶ to 1×10⁻⁴ mol s⁻¹ cm⁻² whenoperating in a 250-700 C temperature range. Furthermore, the moltenhydroxides and the solid support structure provide superiorelectrochemical, thermal, and hydrothermal stability than any systemutilizing organic or transition metal oxide components. The presentlydisclosed inventive systems are therefore superior due to a combinationof 1) high separation rates; 2) operability at appropriate temperatures;3) high selectivity for acidic gases; 4) low energy costs; 5) lowinfrastructure costs; and 6) high stability/durability of the separationmembrane under operational conditions.

Accordingly, in one embodiment, a process for separating acidic gasessuch as oxides of carbon, nitrogen and/or sulfur from emissions mayproceed substantially as shown in FIG. 4 and according to method 400.The method 400 may be performed in any suitable environment, such as apower plant, cement factory, or other suitable emissions sourceincluding a separation membrane as shown in FIGS. 2A-2B, or otherenvironments suitable for the particular type of gas separation, and aswould be understood by a person having ordinary skill in the art uponreading the present descriptions.

Method 400 includes operation 402, in which a gas mixture is exposed toa separation membrane at an elevated temperature (and optionally atelevated pressure). The separation membrane preferably comprises aporous support and at least one molten alkali metal hydroxide disposedwithin pores of the porous support, as described hereinabove accordingto various embodiments. In some approaches, the optional elevatedpressure may cause a pressure gradient across the separation membrane,which may further optionally be balanced by applying a sweep gas to oneside of the membrane.

Of course, as will be understood by persons having ordinary skill in theart upon reading the present descriptions, method 400 may additionallyand/or alternatively include any suitable materials, features oroperations as described herein.

Accordingly, method 400 may include reversibly solvating the acidicgases in the at least one molten alkali metal hydroxide at an inlet ofthe separation membrane, and releasing solvated ions from an outlet ofthe separation membrane, the solvated ions being anionic forms of theacidic gases.

Optionally, applying a potential across the separation membrane mayfacilitate selectivity and permeance of the membrane to the acidicgases, in one approach. Accordingly, the method 400 may include applyingan alternating current (A/C) across the separation membrane.

The operational pressure to which the membrane is exposed is preferablyin a range from about 0.25 atmosphere to about 20 atmospheres, and morepreferably in a range from about 5 atmospheres to about 20 atmospheres.

Alternatively, the total pressure gradient across the membrane could beas low as 0 atmospheres if a sweep gas is used, which is a preferredapproach in some embodiments to minimize energetic cost of separatingthe offensive and/or acidic gases, since applying a vacuum or pressuregradient across the membrane may be relatively energy intensive in someapplications. Accordingly, in various approaches applying a pressuregradient across the separation membrane may actually involve equalizingpressure across the gradient, causing an existing pressure gradientmagnitude to drop to zero magnitude. As such, as discussed herein“applying a pressure gradient” should be understood to includeembodiments where an existing pressure gradient across the membrane isreduced, potentially to zero magnitude, e.g. by applying a sweep gas toone side of the membrane.

The elevated temperature is in a range from about 200 C to about 700 C,more preferably in a range from about 350 C to about 500 C, in variousembodiments.

The acidic gases preferably include one or more gases having a formulaof: CO_(x), NO_(y) and SO_(z), where x is a value in a range from 1-2, yis a value in a range from 1-3, and z is a value in a range from 1-4.

Applications/Uses

Embodiments of the present invention may be used in a wide variety ofapplications, particularly for separation of carbon dioxide from fluegases of fossil fuel power plants, vehicular gas emissions, or any othersuitable source of greenhouse gases that would be understood by a personhaving ordinary skill in the art upon reading the present disclosure.

In addition, the presently disclosed inventive separation membranes andtechniques of use thereof may be employed in other systems andapplications relying on separation, such as fuel cell technology, fuelsynthesis, etc. as would be understood by persons having ordinary skillin the art upon reading the present disclosures.

The inventive concepts disclosed herein have been presented by way ofexample to illustrate the myriad features thereof in a plurality ofillustrative scenarios, embodiments, and/or implementations. It shouldbe appreciated that the concepts generally disclosed are to beconsidered as modular, and may be implemented in any combination,permutation, or synthesis thereof.

In addition, any modification, alteration, or equivalent of thepresently disclosed features, functions, and concepts that would beappreciated by a person having ordinary skill in the art upon readingthe instant descriptions should also be considered within the scope ofthis disclosure.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method for separating acidic gases from a gasmixture, the method comprising: exposing the gas mixture to a separationmembrane at an elevated temperature, wherein the separation membranecomprises a porous support and at least one molten alkali metalhydroxide disposed within pores of the porous support.
 2. The method asrecited in claim 1, comprising reversibly solvating the acidic gases inthe at least one molten alkali metal hydroxide at an inlet of theseparation membrane.
 3. The method as recited in claim 1, comprisingreleasing solvated ions from an outlet of the separation membrane, thesolvated ions being anionic forms of the acidic gases.
 4. The method asrecited in claim 1, comprising applying a potential across theseparation membrane.
 5. The method as recited in claim 1, comprisingapplying an alternating current (A/C) across the separation membrane. 6.The method as recited in claim 1, comprising applying a pressuregradient across the separation membrane, wherein the pressure gradientis in a range from about 0 atmospheres to about 20 atmospheres.
 7. Themethod as recited in claim 1, wherein the elevated temperature is in arange from about 200 C to about 700 C.
 8. The method as recited in claim1, wherein the acidic gases are gases having a formula selected from agroup consisting of: CO_(x), NO_(y) and SO_(z), wherein x is a value ina range from 1-2, y is a value in a range from 1-3, and z is a value ina range from 1-4.
 9. The method as recited in claim 1, comprisingexposing the separation membrane to a sweep gas on a side of theseparation membrane opposite a side to which the gas mixture is exposed,wherein the sweep gas comprises steam.